The emphasis of the proposed research is to examine in-depth capillary endothelial cell swelling in low-flow ischemia at the microcirculatory and whole-organ levels. We have previously shown that this microcirculatory phenomenon evolves in hemorrhagic shock, and the hypothesis is now advanced that it is a universal manifestation of hypoperfused organs. The long-term research objective is to further understand the pathophysiology involved in low-flow ischemia at the vital nutritive capillary level. With reference to capillary endothelial cell swelling in low-flow ischemia, the specific research aims are to: 1. examine the relationship between swelling and perfusion, 2. determine the role of blood pH in swelling, 3. identify the mechanisms for swelling, 4. examine the effects of fluid reperfusion on swelling, and 5. determine the implications of swelling on whole-organ blood flow. To define the extent of endothelial cell swelling in low-flow states, intravital microscopy will be used to visualize capillaries in the rabbit tenuissimus muscle after hemorrhage or proximal arterial occlusion. In hemorrhagic shock, swelling could be due to the diminished capillary perfusion or the induced metabolic acidosis (drop in systemic pH), or a combination of the two. The influence of blood pH will be investigated by maintaining pH after hemorrhage, and alternatively, be decreasing pH with flow unchanged. A causal role of blood pH is supported by the theory that swelling results from intracellular pH regulation through Na+/H+ exchange. To characterize the mechanisms for swelling, the drug amiloride and its analogs will be given prior to an ischemic episode to inhibit transmembrane Na+ pathways. We have found that hypertonic, but not isotonic, saline infusion after shock reverses swollen endothelium beyond expected osmotic shrinkage. Hypertonic mannitol infusion will be tried to explore the possible specific role of the Na+ ion. The impact of conventional blood and iso-osmotic fluids, along with hyperosmotic solutions, will be assessed in those ischemic states in which cell edema is produced. The hemodynamic consequence of cell swelling is that it narrows the capillary lumen, thereby increasing resistance to flow. The effects of narrowed capillaries on whole-organ blood flow during shock and reperfusion will be assessed with a microsphere technique. These proposed studies are intended to give a better understanding of capillary dysfunction in low-flow microcirculatory ischemias germane to clinical medicine, with the ultimate goal of improving therapeutic strategies.